1. Field of the Invention
The present invention relates to an optical observation instrument with at least two optical transmission channels that respectively have one partial ray path and an electronic image recording system for recording, sequentially in time, the partial ray bundles transmitted by the optical transmission channels. An example of such an optical observation instrument is a stereoscopic optical observation instrument such as e.g. a stereo microscope, more particularly a surgical microscope. In such an instrument, the stereo channels are the optical transmission channels, each of which transmitting a stereoscopic partial ray path, i.e. the ray path for a stereoscopic partial image, to the electronic image sensor.
2. Description of the Related Art
The prior art has disclosed a number of arrangements for recording stereoscopic images, in which separate image receivers and, at least in part of the imaging ray path, separate imaging optical systems are provided for the two stereoscopic partial ray paths. By way of example, US 2004/0017607 A1 describes a stereo microscope, which has a beamsplitter common to both stereoscopic partial ray paths and a common main objective. Otherwise, the microscope has optical components and image sensors which are respectively only provided for the partial ray bundle of a stereoscopic partial ray path. Such arrangements have disadvantages as a result of the underlying principle, for example a large installation volume and relatively high costs for providing double the number of optical systems and image sensors.
In addition to the stereoscopic optical observation instruments with a common main objective, which are also referred to as telescope systems, there are also stereoscopic optical observation instruments in which, additionally, use is made of separated main objectives, and so the two stereo channels only have optical components that are completely separated from the components of the respectively other channel. Such systems are referred to as Greenough systems. In order to provide partial images that have been correctly adjusted stereoscopically at different object distances in Greenough systems, the divergence angle between the optical axes of the stereo channels must be adjusted. Moreover, the magnifications in the two stereoscopic partial ray paths must be exactly identical. In the case of a zoom system, this identity of the magnifications must be ensured over the entire zoom range, which places great demands on the production and adjustment. Like in the case of telescope systems with a common main objective and further optical components separated into stereo channels, Greenough-type systems have a large installation volume and high costs for providing double the number of optical components. Moreover, Greenough-type systems have complex mechanical designs.
Furthermore, the prior art has disclosed optical observation instruments, in which the stereoscopic partial ray paths are imaged on a common image receiver by a common main objective, the common image receiver recording the stereoscopic partial images alternately in time. To this end, a device is required that, during a first time interval T1, allows the light in a first partial ray path to pass to the image receiver and at the same time blocks the light in the second partial ray path and, during a subsequent second time interval T2, allows the light in the second partial ray path to pass to the image receiver and at the same time blocks the light in the first partial ray path. Here, the time intervals T1 and T2 correspond to the integration time at the image sensor and are typically of the order of a few milliseconds. The required frequency for switching the channel then typically is 50 to 100 Hz. In order to block the light in the partial ray paths, or to allow it to pass, use is typically made of stops (shutters) that can be switched synchronously with the camera. These stops alternately let light pass through one of two stop openings. By way of example, such a system is described in U.S. Pat. No. 5,828,487.
Shutters may be based on both mechanical and optical principles and are used in the vicinity of pupils in order to unblock partial pupils of a stereo basis alternately and thus allow the observer to see a plastic scene. In the case of mechanical shutters, the passage of light to the image receiver is mechanically blocked in one stereo channel for a predetermined time interval and simultaneously unblocked in the other stereo channel by means of a movable, usually rotating, device. The advantage of mechanical shutters lies in the fact that there are no light losses for the respectively opened stereo channel. However, it is disadvantageous that a mechanical shutter may cause vibrations and noise. This particularly holds true in the case of relatively high switching frequencies. Moreover, ensuring precise synchronicity between shutter and camera requires control with a feedback loop. Furthermore, the inertial mass of the shutter component means that the switching frequency cannot be modified abruptly. Moreover, if there is a desire to produce more than one stereo basis (this may for example be necessary if in addition to the treating medical practitioner as a main observer using the surgical microscope, there also is an assistant as co-observer, whose stereo basis differs from the stereo basis of the main observer by an angle not equal to 180°), two pairs of pupils must be served in succession by the shutter. That is to say, light may only pass through one of four pupils at any one time. Moreover, it is often desirable for the connecting line between the pupils of the one pupil pair to be able to include any angle with the connecting line between the pupils of the other pupil pair, more particularly angles between 10° and 90°, so that the two observers can undertake a surgical intervention in their respective optimum position with respect to the patient. So that the shutter function by means of rotating stops can allow an unambiguous separation of the stereo channels for all orientations of the pupil pairs with respect to one another, the shutter may only unblock a small angle segment <<90° for transmission, while the light from the entire remaining angular region must be blocked. As a result, only a fraction of the theoretically possible integration time per frame on the image sensor can be used, which in turn leads to a loss in image brightness. Thus, overall, rotating stops do not constitute a light-efficient solution for switching channels between a plurality of pupil pairs that are rotated with respect to one another.
In addition to the mechanical shutters, the prior art has disclosed electro-optical shutters. Liquid-crystal stops are an example of these; here the optical transmission of light with a predetermined polarization state can be switched with a high frequency and without movable mechanical components. If the liquid-crystal stop is designed such that portions can be switched to be transparent or non-transparent independently of one another, it is also simple, compared to a solution with mechanical shutters, to switch a plurality of pupil pairs, as is required in the case of more than one observer, without there being a reduction in the theoretically possible integration time per frame on the image sensor. The principle of the liquid-crystal stops is based on the fact that an electric control voltage, applied to a liquid-crystal layer, leads to a polar or chiral orientation of the liquid-crystal molecules and, resulting therefrom, a linear or circular birefringence. If such a liquid-crystal layer is situated in the ray path between a polarizer and an analyzer, oriented in the passage or block direction thereto, an increase in the control voltage brings about a reduction or an increase in the transmission through the entire device, and so the shutter effect can be controlled electronically. However, a disadvantage of liquid-crystal stops is that, as a result of the underlying principle, they are only able to transmit a fraction of the light flow over the stereo channel in the allow-passage setting. Since, as a result of the underlying principle, liquid-crystal shutters can only switch light of one polarization state but most applications use unpolarized light sources, there typically are light losses of at least 50%. However, the actual light losses generally are even greater since the liquid-crystal medium itself only has a restricted transmission of typically 70 to 80%.
In order to avoid high losses by polarization in liquid-crystal stops, there is the option of using liquid-crystal stops which are based on polymers, as disclosed in e.g. EP 0 590 984 A1. In these, liquid crystals are dispersed in a fixed polymer structure. An electric field is used to align the liquid crystals such that domains form between fixedly aligned polymers. This increases the light scattering, and so the stop becomes non-transparent. By changing the applied electric voltage, the stop may be switched between a transmitting and a scattering state. Although such liquid-crystal stops make it possible to avoid the loss of at least half the light, the transmission of such a component is also only 80% at best. Moreover, the light is scattered and not absorbed in the non-transparent state, and so parts of the stray light could reach the image receiver through the optical system. Thus, the use of liquid-crystal displays on polymer basis does not render possible a high-contrast separation of the stereo channels.
Compared to this prior art, the object of the present invention may be considered to be the provision of an advantageous optical observation system with at least two optical transmission channels such as e.g. two stereo channels.